The disclosures of Japanese Patent Application No. HEI 11-287517 filed on Oct. 8, 1999 and No. HEI 11-334100 filed on Nov. 25, 1999 including the specification, drawings and abstract are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates to a fuel cell having an anode and a cathode that sandwich a hydrogen ion-permeable electrolyte layer and, more particularly, to a technology for reducing the size of a fuel cell stack.
2. Description of the Related Art
A fuel cell has an anode and a cathode that sandwich a hydrogen ion-permeable electrolyte layer, and generates an electromotive force by causing reactions as in equations (1), (2) on the anode and the cathode, respectively.
Anode:
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
Cathode:
(1/2)O2+2H++2e31 xe2x86x92H2Oxe2x80x83xe2x80x83(2)
Various types of fuel cells based on different kinds of electrolyte layers, for example, a phosphoric acid fuel cell, a molten carbonate fuel cell, a electrolyte fuel cell, an alkaline fuel cell, etc., have been proposed. Recently, a polymer electrolyte fuel cell adopting a hydrogen ion-conductive polymer membrane as an electrolyte layer is receiving attention because of, for example, its high output density and size reduction potentiality. With regard to the polymer electrolyte fuel cells, various improvements have been and are being considered.
In any of the aforementioned types of fuel cells, the theoretical electromotive force per unit cell is about 1.23 V. Therefore, a desired voltage is achieved by stacking a plurality of cells. A unit formed by stacking cells and securing them through the use of a case is termed stack. In a typical stack, the cell stacking precision appears as an internal resistance. Therefore, if an extremely great number of cells are stacked, the internal resistance becomes large and the fuel cell efficiency decreases. Furthermore, stacking an extremely large number of cells makes it difficult to equally supply fuel gas to the cells. For these reasons, it is a normal practice to avoid constructing a fuel cell apparatus by using a single stack in which cells are stacked to a number that substantially achieves a desired voltage. Instead, plurality of divided fuel cell stacks are connected in series so as to achieve a desired voltage.
As a technology related to this invention, a fuel cell apparatus employing a plurality of stacks is proposed in Japanese Patent Application Laid-Open No. HEI 8-171926. This fuel cell apparatus is able to equally supply fuel to the individual stacks, and allows a size reduction of the entire apparatus. The fuel cell apparatus has a construction in which four stacks are connected via supply/discharge members.
However, it has been found that when fuel cell stacks are to be installed in various appliances, such as vehicles and the like, there are various problems as mentioned below, in addition to the problems in supplying and discharging fuel. According to the conventional art, size reduction of fuel cells is not sufficiently considered in means for solving problems as mentioned above. Therefore, a solution to a problem as mentioned above often gives rise to a problem of a size increase of a fuel cell. In other words, preferable means for solving the below-mentioned problems haven not been thoroughly considered.
A first problem with the fuel cell stack is one attributed to cooling. Fuel cells are cooled by cooling water which flows in cooling water channels formed in each separator that defines gas channels of the corresponding cell. A typical separator is formed by an electrically conductive member. Therefore, due to contact with the electrically conductive separators during the process of cooling cells, cooling water is electrified in accordance with the electric potentials of the electrodes. In a construction having a water-supplying opening for supplying cooling water into a stack and a water-discharging opening for discharging cooling water from the stack, there is a potential difference near the openings. In such a case, the electric potential difference may cause detrimental effects such as galvanic corrosion at the water-supplying and water-discharging openings and the like.
To avoid such detrimental effects, it may be conceivable to, for example, cover the water-supplying and water-discharging openings with an electrically insulating material, or the like. However, this measure results in an increased size of the stack. Particularly in a construction in which a plurality of stacks are connected, the potential difference between the water-supplying and water-discharging openings is as high as several hundred volts. Therefore, in such a construction, it becomes necessary to increase the size of an insulating coating member, so that the effect of the insulating coating members on the size of the apparatus becomes great.
As another measure, it may be conceivable to provide a water-supplying opening and a water-discharging opening at a site where there is no electric potential difference. However, since this measure increases the restrictions regarding the sites of provision of the water-supplying and water-discharging openings, the freedom in designing cooling water channels decreases, thereby impeding the size reduction of the apparatus.
A second problem regarding the stack is attributed to the discharge of water produced by the reactions. As indicated in equations (1), (2), a fuel cell produces water (H2O) through the reactions therein. Water produced in each cell is transported along with gas flows through a manifold for supplying gases to the stack, to a gas-discharging opening. In a polymer electrolyte fuel cell, water for moisturizing the electrolyte membranes is also transported to the gas-discharging opening via the same route as mentioned above. If the amount of water transported to the gas-discharging opening increases, a phenomenon generally termed flooding may occur, causing unstable operation of the fuel cell. More specifically, condensed water droplets formed within the gas-discharging opening reduce the sectional area of the gas-discharging opening and thereby impede gas flow, so that supply of gas to each cell is impeded. This results in unstable power generation.
To avoid such a problem, Japanese Patent Application Laid-Open No. HEI 11-204126 proposes a construction in which a stack is provided with a drain port. However, since this construction includes the drain port and a drain valve provided outside the stack, there is a problem of a great size increase of the stack construction and therefore a great size increase of the entire fuel cell construction. Furthermore, in a fuel cell construction having a plurality of stacks, it is necessary to provide drain mechanisms separately for the individual stacks, so that the size increase of the fuel cell construction becomes even greater.
A third problem regarding the stack is attributed to the cell insulating characteristic. A stack is formed by securing stacked cells in such a manner that the cells do not separate from one another in the stacking direction. An external structure for securing the cells is herein referred to as xe2x80x9cstack case.xe2x80x9d Since the stacked cells are a set of electrodes, it is necessary to insulate the stack case and the stacked cells from each other if a stack is constructed as described above. In a stack related to this invention, an insulator, such as a silicone rubber or the like, is inserted between the stacked cells and the stack case to provide insulation therebetween. However, if the insulation between the stack case and the stacked cells is to be achieved by the above-described construction, the stack production process must include a step of inserting an insulator, so that productivity may decrease. Since in the process of forming a stack by stacking cells, precision related to internal resistance is required, the addition of the insulator inserting step greatly reduces the productivity in some cases. Furthermore, since insulators, such as a silicone rubber and the like, generally have relatively low precision with respect to the thickness thereof, it is necessary that when considering variations in the thickness of the insulator, the stack case be formed with increased dimensions, in order to construct a stack without causing unnecessary loads on the cells. Furthermore, since the insulator needs to have a certain thickness in order to maintain the configuration thereof, the insulator becomes inconveniently large, resulting in a size increase of the stack case.
As a technology for avoiding the aforementioned size increase, Japanese Patent Application Laid-Open No. HEI 8-162143 discloses a technology in which four side surfaces of a stack are coated by applying a rubber thereto. However, if this technology is employed to achieve insulation of a stack, the step of applying rubber must be added. Furthermore, should breakage or the like occur within a rubber-coated stack, repair will be difficult. In view of these circumstances, a technology for reliably insulating a stack and avoiding the size increase of the stack case without reducing the productivity of stacks has been demanded.
A fourth problem of the stack is attributed to securement of water-tightness, dust-tightness, and rigidity of the stack. In a stack, cells are fixed by a stack case as described above. However, it is often the case that the stack case is provided with a not-completely sealed structure in view of the need to mount a terminal for monitoring the voltage across the cells, the operability in mounting the terminal, etc. Therefore, if a stack having such a structure is installed and used in various apparatuses, such as vehicles and the like, there is possibility that water, dust and the like will enter gaps between cells. Furthermore, such apparatuses normally produce vibrations during operation. If such vibrations or loads attributed to the vibrations act on the stack, there is possibility that gaps may be formed between cells due to strains occurring in the stack. Therefore, there is possibility that the stack may suffer a power generating efficiency reduction due to an increase in the internal resistance, and a power generation failure, etc.
To solve these problems, it is possible to employ a method in which the outer peripheral surface of a stack case is completely sealed and the rigidity of the stack case is increased to such a degree that the stack case is not deformed by vibrations or the like. However, the addition of the step of sealing the outer periphery of the stack case degrades the productivity of the stack. In order to sufficiently increase the rigidity of the stack case, it is necessary to increase the plate thickness of the stack case. Therefore, an increase in the rigidity of the stack results in a weight increase and a size increase of the stack. These problems have particularly great effect in a fuel cell apparatus having a plurality of stacks.
A fifth problem regarding a stack is a problem attributed to a mechanism for applying an elastic force to the stacked cell. When a stack is formed by stacking cells, it is desirable to place cells as close to one another as possible, in order to reduce the internal resistance. Furthermore, during power generation, heat is produced through chemical reactions, and thermally expands the cells. Therefore, if stacked cells are completely fixed, there is possibility of deformation due to heat strains, and therefore there is possibility of causing detrimental effects such as a power generation failure, a service life reduction, etc.
A technology for solving this problem is disclosed in Japanese Patent Application Laid-Open No. HEI 11-233132. In this technology, an end plate is attached, via a disc spring, to an end of stacked cells. Due to the elastic force of the disc spring, deformation caused by thermal expansion or the like is absorbed, and the cells are forced into close contact with one another. Another technology for solving aforementioned problem is disclosed in Japanese Patent Application Laid-Open No. HEI 7-335243. In this technology, an end plate is attached, via an elastic member, to an end of stacked cells, and a space between the end plate and the end of the stacked cell is used as a pressure chamber into which liquid can be charged. Using the elastic force of the elastic member and the pressure of the liquid, the technology absorbs deformation caused by thermal expansion or the like and applies a force to place the cells in close contact with one another.
In these technologies, however, the end plate is fixed by bolts that extend through cells in the stacking direction. Therefore, there is a problem of a bolt space increasing the size of the stack and, in particular, the dimension of the stack in the stacking direction. In a fuel cell, it is necessary to stack many cells in order to secure a certain voltage, so that the dimension of the stack in the stacking direction inevitably tends to increase. It is often preferable to avoid a configuration of a fuel cell having an extremely large size in any given direction, in view of installation space thereof in vehicles and other various apparatuses. Therefore, it is desirable to reduce the size of a cell in the stacking direction. A size increase attributed to the bolt space mentioned above reduces the efficiency in installing the stack in an apparatus. The effect of an increase in the size in the stacking direction is particularly great in a fuel cell apparatus having a plurality of stacks. Therefore, there has been a demand for a technology capable of providing an appropriate elastic force to stacked cells in a stacking direction and capable of reducing the size of the stack and, in particular, the size thereof in the stacking direction.
As stated above, the conventional fuel cells have various practical problems. Due to these problems, the significant problem of a large-size stack exists.
Accordingly, it is an object of the invention to solve at least one of the aforementioned five problems while avoiding a size increase of a stack.
In order to solve at least one of the aforementioned various problems while achieving a size reduction of a fuel cell apparatus, the invention adopts constructions as follows.
In accordance with a first aspect of the invention, a fuel cell apparatus includes at least one cell stack formed by stacking unit cells, and a cooling mechanism that cools the at least one cell stack by passing a cooling medium through the at least one cell stack. The cooling mechanism has a supply opening for supplying the cooling medium to the cell stack, and a discharge opening for discharging the cooling medium from the cell stack. The supply opening and the discharge opening are electrically short-circuited.
In the first aspect of the invention, the supply opening and the discharge opening are short-circuited, so that the electric potential difference in the cooling water between the supply opening and the discharge opening can be eliminated. Therefore, detrimental effects, such as galvanic corrosion and the like, can easily be avoided.
The means for electrically short-circuiting the cooling medium passage having an electric potential difference can be realized by connecting the supply opening and the discharge opening via an electrically conductive member, so that detrimental effects, such as a size increase of the fuel cell apparatus, a production cost increase, etc., will not be caused. Furthermore, it becomes unnecessary to provide an insulating member for the supply opening and the discharge opening, and therefore a size increase of the apparatus can be avoided. Furthermore, a restriction that the supply opening and the discharge opening must be provided at sites without an electric potential difference is also eliminated. Therefore, the degree of freedom in design increases, so that the apparatus can be further reduced in size.
The supply opening and the discharge opening are provided in the cell stack in order to supply and discharge the cooling medium. The supply opening is a portion where the cooling medium is supplied to the cell stack, and includes a cooling medium passage upstream of the supply opening. The discharge opening is a portion where the cooling medium is discharged from the cell stack, and includes a cooling medium passage downstream of the discharge opening. Therefore, the short circuit device in the invention can be provided outside the cell stack. Hence, it becomes possible to mount the short circuit device after the cell stack is formed. Thus, this construction advantageously makes it possible to provide the short circuit device without degrading the efficiency in production of the cell stack. Furthermore, if a problem, such as a broken wire or the like, occurs, the problem can be easily coped with.
The short circuit device may be provided for a single cell stack. However, if the fuel cell apparatus has a plurality of cell stacks, a cooling medium passage may be provided which conveys at least a portion of the cooling medium upstream of the supply opening of each cell stack and at least a portion of the cooling medium downstream of the discharge opening of each cell stack, and the short circuit device may be provided at a location where a common cooling medium passage shared by the cell stacks is provided.
In this construction, the short circuit device is provided for a common cooling medium passage, so that the electric potential difference in the cooling medium can be eliminated without a need to provide a short circuit device for each cell stack. Therefore, the process and cost for providing the short circuit device can be minimized. Since the cooling medium can have a considerably great electric potential difference after passing through cell stacks, the usefulness of the invention is very high in that the electric potential difference can easily be eliminated.
As an example of the fuel cell apparatus having a plurality of cell stacks, a construction having a supply/discharge member that realizes the supply and discharge of a fuel between the cell stacks and an external device by performing the function of distributing the fuel gas supplied to the supply/discharge member to the cell stacks and the function of gathering an exhaust gas from the cell stacks may be cited. In this case, the aforementioned common cooling medium passage is formed within the supply/discharge member, and the construction of the invention can be realized by short-circuiting the vicinities of the supply opening and the discharge opening for supplying and discharging the cooling water with respect to the supply/discharge member.
In accordance with a second aspect of the invention, a fuel cell apparatus includes a plurality of cell stacks formed by stacking unit cells, and a supply/discharge member that supplies and discharges a fuel between the cell stacks and an external member by performing a function of distributing a fuel gas supplied to the supply/discharge member to the cell stacks and a function of gathering an exhaust gas from the cell stacks. The supply/discharge member has, as internal constructions, a gathered gas channel through which the gathered exhaust gas flows, and a drainage member that is branched from the gathered gas channel and that discharges water droplets from the gas channel.
In the second aspect, the drainage mechanism provided in the supply/discharge member appropriately discharges water droplets from the gas passage, thereby avoiding flooding. Furthermore, since the drainage mechanism is provided in the supply/discharge member, the apparatus eliminates the need to provide a drain valve or the like outside the supply/discharge member, and therefore avoids a size increase of the apparatus, unlike the related-art technology described in Japanese Patent Application Laid-Open No. HEI 11-204126. In particular, since the drainage mechanism is provided in the supply/discharge member, provision of a drainage mechanism for each cell stack is avoided, so that the apparatus size can be reduced.
The drainage mechanism may include a water storage mechanism for temporarily storing water droplets, and a drain pipe for discharging water from the water storage mechanism. The drain pipe may be constructed so that water is discharged by gravity. The drain pipe may also be constructed so that water is discharged by using the pressure of gas flowing through the gas channel. In order to smoothly and consistently supply the fuel gas to the unit cells, the gas is supplied at a relatively high pressure. Although there is a pressure loss in the unit cells, the gas discharged therefrom has a pressure that is sufficiently higher than the atmospheric pressure that it is possible to discharge water through the use of the exhaust gas pressure, by using a water storage mechanism in which the exhaust gas pressure acts on water surfaces. For example, a water storage mechanism may be provided in a site of the gas channel where the pressure becomes locally high due to a curvature of the passage, a water storage mechanism may be provided together with a branch channel connected to the gas channel at an acute angle to the exhaust gas flowing direction, or the like. If the pressure is used for drainage, the degree of freedom regarding the position of the drain pipe increases, so that the apparatus size can be reduced in comparison with drainage based merely on the gravity.
In accordance with a third aspect of the invention, a fuel cell apparatus includes a plurality of cell stacks formed by stacking unit cells, and a securing member for securing the stacked unit cells. The securing member has an insulating layer that is provided on a surface of the securing member that contacts the unit cells.
In the third aspect, a step of inserting an insulating member between unit cells and a securing member can be omitted from the process of producing a cell stack. Thus, productivity improves. Since the process of stacking unit cells is a precision process that greatly affects the performance of the cell stack, simplification of this process leads to a considerable improvement in productivity. The method for providing the insulating layer integrally with the securing member may be, for example, one in which the insulating member is adhered to a surface of the securing member, one in which an insulating material is applied to a surface of the securing member, and the like. Since the insulating member and the securing member are integrally formed by a method as mentioned above, it becomes possible to reduce the thickness of the insulating layer, and it also becomes possible to curb the dimensional error in thickness, in comparison with a case where the insulating layer is separately prepared.
Furthermore, if the insulating layer is separately prepared, it is necessary to provide a sufficiently large clearance between the cell stack and the securing member so that should the insulating member shift in position, the securing member will not contact the cell stack. However, since the insulating layer is formed integrally with the securing member, such a consideration becomes unnecessary, and the clearance between the unit cells and the securing member can be reduced. Due to the effects mentioned above, the fuel cell apparatus in accordance with the third aspect, in which the securing member and the insulating layer are integrated, allows a size reduction of the apparatus.
In accordance with a fourth aspect of the invention, a fuel cell apparatus includes a plurality of cell stacks formed by stacking unit cells, and a container that houses the plurality of cell stacks altogether, and that has a sealed construction that is able to prevent penetration of foreign substances from outside. Examples of the foreign substance include dust, water, and the like.
The provision of the container eliminates the need to provide each cell stack with a device for completely blocking foreign substances. Therefore, the construction of the cell stack can be simplified, and the cell stack can be reduced in size. Furthermore, productivity can be improved, and the production cost can be reduced. Furthermore, if there is a need to monitor the electric potential of the unit cells, the provision of the container makes it possible to construct the cell stack in a condition where the unit cell scan be seen. Thus, the container is highly useful.
The fuel cell apparatus in the fourth aspect of the invention also has advantages in view of securing rigidity. If a fuel cell apparatus is installed in a vehicle or the like, the fuel cell apparatus is subject to vibrations and various external forces. In order to perform stable power generation, it is necessary to secure such rigidity for the fuel cell apparatus that the fuel cell apparatus does not undergo deformation due to vibrations or external forces. The xe2x80x9cdeformationxe2x80x9d herein is mainly a bending deformation and a torsional deformation. The rigidity regarding these deformations can be evaluated using second moment of area and polar moment of area as indices. These factors are known to become greater in a section that has a greater distance from the neutral axis of the bending deformation and the rotating axis of the torsional deformation. Since the container in the fourth aspect houses the cell stacks as a unit, the container apparently has a greater second moment of area and greater polar moment of area than the cell stacks. Therefore, the fuel cell apparatus in the fourth aspect is able to secure a sufficient rigidity while curbing increases in the plate thicknesses of the members. If the container has a good rigidity, the cell stacks do not need to have such a great rigidity, so that a size reduction can be achieved. Furthermore, since an increase in the plate thicknesses of the container can be curbed, an increase in the total weight of the fuel cell apparatus can be curbed.
In the fuel cell apparatus in the fourth aspect may further include a discharge mechanism that discharges at least one of a liquid and a gas present in the container, to outside the container, separately from a supply/discharge mechanism that supplies and discharges a fuel gas, an oxidative gas, and cooling water with respect to the cell stacks within the container.
Since hydrogen, which is used as a fuel gas, is a substance of a very small molecule size, there are cases where hydrogen leaks from various joints of unit cells in an exuding manner during operation. Water produced through the reactions in the fuel cell apparatus may also leak out of the cell stacks. In the fuel cell apparatus in this aspect, there is a possibility that a liquid or a gas discharged as described above will accumulate in the container, since the container is sealed. However, the discharge mechanism as described above is able to appropriately discharge the gas or liquid from the container. As the discharge mechanism, a simple construction wherein a discharge pipe is connected may be used. However, in order to prevent entrance of external substances, it is preferable to provide a valve body or the like in the connecting portion.
In accordance with a fifth aspect of the invention, a fuel cell apparatus includes at least one cell stack formed by stacking unit cells, an elastic member that applies an elastic force to the unit cells in a stacking direction, a pair of end plates which are disposed at opposite ends of the unit cells stack, in such a manner that the end plates are substantially parallel to the unit cells, and which have such a rigidity that the end plates are allowed to be regarded as rigid plates with respect to the elastic force, and an interconnecting member which interconnects the end plates, and which causes on the end plates a force that counterbalances the elastic force. The end plates and the interconnecting member are fastened by a fastening member inserted in a direction substantially perpendicular to the stacking direction.
In the fifth aspect, the elastic force from the elastic member makes it possible to absorb thermal deformation and hold the unit cells in sufficiently close contact with one another, thereby realizing stable operation. In the fifth aspect, the mechanism for applying an elastic force as described above adopts a construction in which the interconnecting member bears the load applied to the end plates as a reaction to the elastic force applied to the unit cells. The fastening member for fastening the interconnecting member and the end plates is inserted in a direction perpendicular to the stacking direction. If the fastening member is inserted in the stacking direction, the size of the cell stack in the stacking direction correspondingly increases. In contrast, if the fastening member is inserted in a direction perpendicular to the stacking direction, such a size increase can be avoided.
The above-described construction is particularly useful in a fuel cell apparatus having a plurality of cell stacks, as stated below. In the fuel cell apparatus having a plurality of cell stacks, it is preferable to provide a supply opening and a discharge opening in the stacking direction, in order to uniformly supply a fuel gas and the like to the unit cells. In particular, if the cell stacks are connected by using a supply/discharge member as described above, each cell stack is connected to the supply/discharge member via one of the end plates of the stack. In the fifth aspect, since the fastening member is inserted in a direction perpendicular to the stacking direction, interference of the fastening member at a junction surface of the supply/discharge member can be avoided. Furthermore, it is possible to check the fastened state achieved by the fastening member even after the supply/discharge member and the cell stacks are connected. Therefore, ease of maintenance improves.
Furthermore, if the fuel cell apparatus of the fifth aspect of the invention has a plurality of cell stacks, it is desirable that the cell stacks be arranged in a direction perpendicular to the direction of insertion of the fastening member. This arrangement avoids interference between the fastening members of adjacent cell stacks, and makes it possible to pursue a further size reduction of the fuel cell apparatus. Furthermore, ease of maintenance also improves. Although the construction according to the fifth aspect is particularly useful where a plurality of cell stacks are incorporated, the construction of the fifth aspect may also be effectively applied to a single cell stack.
Although the fuel cell apparatuses in accordance with the first to fifth aspects of the invention have been separately described above, it is also possible to construct various fuel cell apparatuses in accordance with combinations of those aspects of the invention. In such a case, a fuel cell apparatus with a combination of the advantages of the fuel cell apparatuses of the first to fifth aspects can be realized. Although the above-described fuel cell apparatuses of the invention are preferably applied to a polymer electrolyte fuel cell whose size reduction is expected, the application is not limited so. That is, the invention is also applicable to various other types of fuel cells, such as phosphoric acid fuel cells, molten carbonate fuel cells, electrolyte fuel cells, alkaline fuel cells, etc.